The invention relates to digital television receivers for vestigial-sideband (VSB) digital television (DTV) signals and, more particularly, to the suppression of interference with DTV signal reception caused by the frequency-modulated audio carrier of a co-channel NTSC analog television signal.
The VSB DTV signals that are broadcast in the United States are transmitted in the same channels used for broadcasting NTSC analog television signals. Generally speaking, the VSB DTV signals are broadcast at lower power than NTSC analog television signals are. Until such time as DTV broadcasting completely supplants NTSC analog television broadcasting, then, there is substantial possibility of co-channel NTSC interference with VSB DTV signal reception, particularly during the summer months. The video carrier modulation components of a co-channel NTSC signal overlap a VSB DTV signal in frequency, which makes the suppression of these video components by frequency selective filtering a difficult task. The current content of the video portion of a co-channel NTSC signal can be predicted quite well from its previous content, however, which facilitates comb filtering to suppress artifacts of the video portion of a co-channel NTSC signal that accompany baseband symbol code detected from the received VSB DTV signal. Prediction of the current content of the audio portion of a co-channel NTSC signal from its previous content is generally more difficult, although short-time prediction of the audio portion is possible based on considerations of continuity in a narrowband signal. The audio carrier modulation of a co-channel NTSC signal does not overlap a VSB DTV signal in frequency, which makes it more feasible to employ frequency-selective filtering to suppress the audio portion of a co-channel NTSC signal than to suppress the video portion.
There has been great concern with carefully controlling the overall amplitude and phase characteristics of the VSB DTV receiver in order to minimize intersymbol error, while at the same time rejecting interference from signals in adjacent channels. Generally, the overall response of the receiver is defined by surface-acoustic-wave (SAW) filtering done using gallium-arsenide devices in internediate-frequency amplifiers for the ultra-high-frequency (UHF) band or using lithium-niobate devices in intermediate-frequency amplifiers for the very-high-frequency (VHF) band. Getting flat amplitude response within xc2x11 dB over a bandwidth of 5.5 to 6 MHz, while maintaining acceptable group delay characteristics, requires SAW filtering with a great number of poles and zeroes to define the receiver bandwidth. It is difficult and expensive to implement such SAW filtering for a VHF band, such as 41-47 MHz. Also, the insertion loss is quite high in a VHF band, typically 15-17 dB for the 41 to 47 MHz band. The SAW filtering to define receiver bandwidth is more easily implemented for a UHF band, such as at 917-923 MHz, as long as care is taken to drive the SAW filter from the optimal source impedance specified by its manufacturer. This is because the xcex94f/f ratio of 6 MHz to 920 MHz is substantially lower than the xcex94f/f ratio of 6 MHz to 44 MHz. Insertion losses also tend to be lower in a UHF band, typically 10-12 dB for the 917 to 923 MHz band.
The cost of a SAW filter used in a UHF or VHF I-F amplifier is substantially increased if it is designed to provide trap filtering for the modulated audio carrier of the co-channel NTSC signal. Since the co-channel NTSC signal is 250 kilohertz from the edge of the television channel, and since critical VSB DTV information extends to within 310 kilohertz of that edge of the television channel, the SAW filter response must reach substantial attenuation in less than a 60 kilocycle range. This is a very difficult requirement to fulfill.
Consequently, trap filtering for the modulated audio carrier of the co-channel NTSC signal is omitted in some DTV receiver designs, and the artifacts of co-channel interference from this carrier are suppressed to some extent by comb filtering performed in the digital regime for reducing artifacts of co-channel interference from the video portion of the NTSC signal. Co-channel interference from the audio portion of the NTSC signal is not suppressed very much unless the comb filtering used to suppress co-channel interference from the video portion of the NTSC signal subtractively combines samples differentially delayed by twelve symbol epochs. Often, however, comb filtering of different type would better suppress co-channel interference from the video portion of the NTSC signal.
So cheaper and better filtering for suppressing co-channel interference from the audio portion of the NTSC signal is a desideratum. It is pointed out in this specification that such filtering is possible in DTV signal receivers in which a low-band final I-F signal with its uppermost frequency in the low or mid high-frequency band is generated and synchrodyned to baseband for recovering symbol code. RLC analog filtering of such an I-F signal can be done to suppress co-channel interference from the audio portion of the NTSC signal. Examples of suitable RLC analog filtering are Butterworth-Thomson transition filters, bridged-T trap filters and bifilar-T trap filters.
Non-uniform group delay attends RLC analog filtering, so designers eschew LC analog filtering in favor of digital filter designs with which uniform group delay can be easily obtained. The non-uniform group delay from LC analog filtering can be substantially compensated by non-uniform group delay designed into SAW filtering used for the VHF I-F signal or SAW filtering used for the UHF I-F signal, however. The inclusion of such delay compensation into such a SAW filter is not expensive. Remaining non-uniformity in group delay is easily remedied by the adaptive channel equalization filtering customarily incorporated into DTV signal receivers. So combinations of LC analog filtering low-band intermediate-frequency DTV signals with preceding SAW filtering of higher-intermediate-frequency DTV signals designed to result in overall group delay that is reasonably uniform can provide the desired cheaper and better filtering for suppressing co-channel interference from the audio portion of the NTSC signal.
RLC filters with maximally flat amplitude characteristics are described by S. Butterworth in a paper xe2x80x9cOn the Theory of Filter-Amplifiersxe2x80x9d in EXP. WIRELESS AND WIRELESS ENG., Vol. 7, p. 536, October 1930. RLC filters with maximally flat delay characteristics are described by W. E. Thomson, M. A., in a paper xe2x80x9cNetworks with Maximally-Flat Delayxe2x80x9d in WIRELESS ENGINEER, Vol. 29, pp. 256-263, October 1952. RLC filters with characteristics intermediate to those of Butterworth and Thomson filters, which transitional filters have better transient response characteristics than Butterworth or Thomson filters are described by Y. Peless and T. Murakami in a paper xe2x80x9cAnalysis and Synthesis of Transitional Butterworth-Thomson Filters and Bandpass Amplifiersxe2x80x9d in the March 1957 issue of RCA REVIEW pp. 60-94. The bridged-T trap filter is described by F. E. Terman, Sc. D., in RADIO ENGINEERSxe2x80x9d HANDBOOK, 1st. Ed., xc2xa713, xc2xa7xc2xa77, pp. 918-920, copyright 1943 to McGraw Hill Book Co., Inc. of New York and London. The procedures for designing bifilar-T traps are known to television engineers from a licensee bulletin LB-961 titled xe2x80x9cAn Analysis of the Bifilar-T Trap Circuitxe2x80x9d supplied by Radio Corporation of America through its Industry Service Laboratory to television receiver licensees on Sep. 16, 1954.
The conversion of very-high-frequency intermediate-frequency VSB DTV signal to a band below 10 MHz for digitization and its subsequent demodulation in the digital regime are described in U.S. Pat. No. 5,479,449. This patent entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVERxe2x80x9d issued Dec. 26, 1995 to C. B. Patel and A. L. R. Limberg. Demodulation in the digital regime is performed in U.S. Pat. No. 5,479,449 by converting the digitized internediate-frequency VSB DTV signal to complex form, to be multiplied in a complex digital multiplier by a complex digital carrier signal supplied from look-up tables stored in read-only memory (ROM). To facilitate converting the digitized I-F signal to complex form using a digital Hilbert transform filter, the final intermediate-frequency band is offset a megahertz or so from zero frequency, but its uppermost frequency is kept in the lower portion of the high-frequency (HF) band extending from 3 to 30 MHz.
Equalization of the digitized baseband symbol coding that results from demodulation is facilitated by choosing a sampling clock of a rate that is related to symbol rate by a whole number ratio and that will satisfy the Nyquist criterion. Supplying the complex digital carrier signal from ROM is facilitated by choosing the carrier in the digitized I-F signal to be a submultiple of the system clock signal rate as described by C. B. Patel and A. L. R. Limberg in U.S. Pat. No. 5,606,579 issued Feb. 25, 1997 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVERxe2x80x9d. Such choice of carrier permits perfect wrap-around of cycles of digital carrier when they are conceived as being mapped to the surface of a cylinder with circumference measured by ROM addresses according to a modular arithmetic.
C. B. Patel and A. L. R. Limberg advocate the digital carrier being located at the upper-frequency end of the final I-F signal band in U.S. Pat. No. 5,731,848 issued Mar. 24, 1998 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING NG FILTERS, AS FOR USE IN AN HDTV RECEIVERxe2x80x9d. That is, the vestigial sideband is above full sideband in frequency in the final I-F signal that is digitized. U.S. Pat. No. 5,731,848 discloses there is reason for this choice of carrier, aside from facilitating the use of Ng filters for converting the real final I-F signal to complex form after its digitization. Fast changes in symbol values are converted to lower-frequency variations in the final I-F signal offered for digitization, which alleviates problems of accurately sampling the final I-F signal as the initial step in digitization. Small changes in sampling phase result in larger changes in the zero-frequency demodulated carrier, so there is tighter automatic frequency and phase control (AFPC) of a local oscillator used in converting the radio-frequency (R-F) VSB DTV signal to the final I-F signal.
The inventors discern that selective filtering of an I-F signal to suppress any co-channel NTSC audio carrier component of the I-F signal accompanying its DTV signal component is substantially easier if the frequency of the I-F signal is as low as possible. In a DTV receiver that converts VHF I-F signal to a low-band I-F signal with uppermost frequency in the lower portion of the high-frequency (HF) range extending from 3 to 30 MHz, the formidable problem of trapping co-channel NTSC audio carrier component in a UHF or VHF I-F signal can be simply avoided, the inventors point out. This is done by selectively filtering the low-band I-F signal for suppressing any co-channel NTSC audio carrier component therein. Since the xcex94f/f ratio of the few tens of kilohertz xcex94f range over which the filter must exhibit cut-off is a smaller fraction of the frequency f at the center of passband,.a SAW filter that traps co-channel NTSC audio carrier component of the low-band I-F signal without severely affecting its DTV signal response can be practically constructed. Alternatively, resistance-inductance-capacitance (RLC) analog filtering can be used for trapping the co-channel NTSC audio carrier component of the low-band I-F signal.
A digital television receiver embodying the invention converts very-high-frequency intermediate-frequency digital television (DTV) signal to a low-band intermediate-frequency signal with uppermost frequency in the lower portion of the high-frequency (HF) range extending from 3 to 30 MHz , selectively filters the low-band intermediate-frequency signal to suppress any co-channel NTSC audio carrier accompanying the DTV signal, and synchrodynes the selectively filtered low-band intermediate-frequency signal to baseband for recovering symbol code.
Certain digital television receivers embodying the invention employ RLC analog filtering for suppressing the frequency-modulated audio carrier of co-channel interfering NTSC analog television signal in the final I-F signal that is synchrodyned to baseband for recovering symbol code. The delay distortion introduced by this analog filtering is compensated for in substantial part in surface-acoustic-wave (SAW) bandpass filtering of the very-high-frequency intermediate-frequency digital television signals. Remnant delay distortion is reduced by channel equalization filtering carried out in the digital regime.
Other digital television receivers embodying the invention employ a surface-acoustic-wave filter for the low-band intermediate-frequency signal, to suppress any co-channel NTSC audio carrier component of this low-band I-F signal accompanying the DTV signal component of this low-band I-F signal.